1. Field of the Invention
The present invention relates to a polishing pad used when flattening an inter-layer insulating film by the chemical mechanical polishing (CMP) method in a process for manufacture of a semiconductor device and to a polishing apparatus and a polishing method using this polishing pad.
2. Description of the Related Art
In recent years, semiconductor integrated circuits have been advancing to the next generation of miniaturization and integration in as little as three years. The design rule has been reduced to 70% of the previous generation and along with the reduction, an increase in the speeds of semiconductor devices has been realized. In order to miniaturize semiconductor devices, for example, it has become necessary to reduce the gate width of the gate electrodes of the transistors and the area occupied by the capacitors in DRAM's and the like and to similarly miniaturize the interconnection portions such as by adopting a multilayer interconnection structure. Further, it has become important to form the contact holes and the like to similarly have miniature openings. Also, along with devices such as the transistors and the capacitors becoming more complicated in structure and three-dimensional, the inter-layer insulating films have become thicker.
The above miniaturization has been achieved by the advances made in miniaturization technology in the process of manufacture of semiconductor devices, in particular the improvement of the resolving power in the lithography process--that is, the technique for using light to transfer a circuit pattern on to a photosensitive organic film (photoresist) coated on a wafer surface. Specifically, the wavelength of the light source used in the lithography process has been shortened. For example, g-rays (436 nm) or i-rays (365 nm) are used for transferring patterns for semiconductor integrated circuits of the 1.0 to 0.5 .mu.m rule, while i-rays are mainly used for transferring patterns of the 0.35 .mu.m rule. Further, for the manufacture of semiconductor integrated circuits of the 0.25 .mu.m rule and beyond, a technique for exposure by using a KrF excimer laser (248.8 nm) or an ArF excimer laser (193 nm) has been studied.
As described above, the improvement of the resolution in the lithography process is on the one hand bringing about a reduction of the depth of focus (DOF) of exposure in the lithography process. Realization of improvement of DOF will first require improvement of the performance of the resist, but demands for miniaturization are currently being made in advance of improvements in the resist performance under existing circumstances. Therefore, a method for reducing the difference of height in the structure of devices as much as possible so as to make up for the insufficient depth of focus when performing the lithography process so as thereby to achieve reliable resolution of miniature patterns without causing focal deviation is being studied.
In this regard, as a method for flattening the height differences in a device structure, recently use has been made of the chemical mechanical polishing method to give the silicon wafer a mirror surface finish. FIG. 1 is a schematic view of a general chemical mechanical polishing apparatus used in the related art for performing this chemical mechanical polishing. This apparatus is roughly constituted by a rotating polishing plate shaft 1, a polishing pad 2, a polishing plate 3 which is supported by the shaft 1 and on whose surface the pad 2 is adhered, a dresser 101 comprised of diamonds 102 etc. electrodeposited on a metal plate and for dressing the surface of the polishing pad 2, a carrier 5 for holding a processed substrate 4 on which an inter-layer insulating film or other layer for polishing is formed (hereinafter also referred to as a "wafer") by a wafer backing film 14, and a polishing slurry delivery system 7 having a polishing slurry delivery nozzle 6 for delivering the polishing slurry 10 onto the polishing pad 2.
After dressing (grinding) the polishing pad 2 by a dresser 101, the polishing plate shaft 1 and a carrier shaft 8 are made to rotate and the wafer 4 is made to press against the top of the polishing pad 2 to polish the wafer 4 by a polishing pressure adjustment mechanism 9 while delivering the polishing slurry 10 to the center portion of the polishing pad 2 from the polishing slurry delivery nozzle 6.
The chemical mechanical polishing method described above, however, suffers from the problems that microscratches occurs in the insulating film of the wafer or other polished layer and that the variations in the polishing rate and the variations in the amounts of polishing in the plane of the wafer are large.
In order to suppress the occurrence of the microscratches, it is necessary to eject the dressing dross of the polishing pad 2 and the diamonds of the dresser generated at the time of dressing of the polishing pad 2, the pieces of the inter-layer film and wafer, and the used polishing slurry, etc. (hereinafter these will also be referred to generally as "impurities") to the outside of the polishing pad 2.
Therefore, in the chemical mechanical polishing apparatus of the related art, the measure has been taken of continuously delivering a sufficient amount of the polishing slurry to the center portion of the polishing pad 2 during the polishing work so as to remove or flush away these impurities from the polishing pad 2 by this polishing slurry.
The reduction of the variations in the polishing rate and the variations in the amounts of polishing in the plane of the wafer require that the following approach be adopted. The principle of chemical mechanical polishing is to form a so-called shallow dressed layer at the surface of the polishing pad 2 by making innumerable scratches on the surface of the polishing pad 2 by a dresser and to polish the wafer 4 in a state with the polishing slurry 10 held in the same so as to enable sufficient delivery of the polishing slurry to the polishing surface of the wafer 4 pressed against the polishing pad 2 and thereby to enable polishing. Taking this into account, the practice has been to dress the surface of the polishing pad by the dresser sufficiently so as to give a sufficient depth and density of the dressed layer and to sufficiently deliver the polishing slurry 10, which also serves as the measure to prevent microscratches, and ensure that the polishing slurry 10 reaches the surface of the wafer 4. This approach constitutes the means for the reduction of the variations in the polishing rate and the variations in the amount of polishing in the plane of the wafer.
However, when forming the dressed layer on the pad surface by dressing in this way and delivering the polishing slurry to polish a wafer, the polishing slurry is pushed out due to the centrifugal force caused by the rotation of the polishing pad and by pressing the wafer against the polishing pad. Almost all ends up being ejected to the outside of the polishing pad without directly contributing to the polishing. Therefore, the expensive polishing slurry ends up being wasted. For this reason, as shown in FIG. 2, in the related art, experiments have been made to cut for example lattice-like grooves 21 in the polishing pad 200 and store the polishing slurry there to increase the opportunity for contact to the wafer.
In the above structure, however, there was a problem in that the polishing slurry ended up being easily ejected to the outside of the polishing pad. In order to solve this problem, a polishing pad out with concentric circular grooves 22 about the center of the polishing pad 200 as shown in FIG. 3 was considered. However, even with this, in actuality, the problems remained that the polishing slurry held in the grooves 22 remained only at the outer circumferences of the grooves due to the centrifugal force and otherwise ended up being ejected to the outside of the polishing pad and that the position at which the polishing slurry held in the concentric circular grooves 22 of the polishing pad started to flow out over the grooves was not constant and therefore the polishing slurry did not always flow out to the front of the wafer.
The fact that the polishing slurry is not effectively utilized and ends up being ejected to the outside of the polishing pad in this way raises the cost of the chemical mechanical polishing. At the same time, since the polishing slurry does not sufficiently contribute to the polishing, there is the apprehension of a reduction of the quality of polishing such as the occurrence of microscratches on the polished surface or variations in the amount of the polishing.